1. Field of Invention
The present invention relates to a search circuit, and more particularly to a segmented search line circuit device for content addressable memory.
2. Description of Related Art
Low power consumption circuit design plays an important role in the modern integrated circuits. Especially in memory circuit design, decreasing the number of transistors to reduce power consumption is important. As network communications and Internet Protocol version 6 (Ipv6) develops, the performance of the content addressable memory determines the searching speed and power consumption of the network chip. In the research of the content addressable memory, the critical object focuses on improving the performance of the match line and the power consumption to reduce the cost and enhance the efficiency. Recently, the power consumption of the match line has been reduced significantly, so most power consumption is caused by the search line of the content addressable memory.
Reference is made to FIG. 1, wherein the match line is divided into two blocks, a first block 101 and a second block 102, in order to reduce power consumption. The block 102 has a global search driver 103, a local search driver 104, a global search line 105 and a local search line 106. The function of the drivers 103 and 104 controls the search line of the block 102 to determine whether or not the search should continue depending on the match result of the block 101. Because the search activity of the second block 102 is determined by the match result of the first block 101, the local search driver 104 sends the data of the global search driver 105 into the local search line 106 only if just one match result of the first block 101 matches. Contrarily, the local search driver 104 does not send the data of the global search driver 105 into the local search line 106 if all the match results of the first block 101 do not match. Through this method, the power consumption caused by the switching activity can be reduced effectively.
However, the amount of registers and the logic gates (ex: or gate) are increased in this way such that the power consumption is raised and the match line efficiency is relatively deteriorated. Moreover, the match result of this kind of design cannot be produced in a cycle, so the pipelined timing control affects the match line efficiency when the match line is applied to the content addressable memory.
Reference is made to FIG. 2, which illustrates another search line solving power consumption with the charge reuse method. In FIG. 2, the capacitor 203 denotes the loading capacitance of the search line (SBLP and SBLN). The control circuit 201 compares the external data with the search line (SBLP and SBLN) data when the external data is input into the control circuit 201. If the data are different from each other, the control circuit 201 drives the transmission gate 202 to make the potentials of the search line (SBLP and SBLN) become the same. When the potentials of the search line (SBLP and SBLN) become the same, the transmission gate 202 shuts down and the control circuit 201 sends the data into the search line (SBLP and SBLN). Through this method of charge sharing, wasted charges caused by charging/discharging of the search line (SBLP and SBLN) are diminished to reduce power consumption. However, this method reduces the power consumption for search lines, but the loading capacitance (CLP and CLN) of the capacitors 203 in the search line is very large. Therefore, the extra control circuit 201 is demanded and consumes extra so that the power consumption of the entire system cannot be reduced. Consequently, although adding the control circuits can reduce the power consumption of the search lines, this method also generates additional power consumption and even affects the efficiency of the match line.